Hardwood plywood is defined in the ANSI/HPVA HP-1-2000standard as, “A panel composed of an assembly of layers or plies of veneer or veneers in combination with a lumber core, particleboard core, MDF core, hardboard core, or of special core material joined with an adhesive. Except for special constructions, the grain of alternate plies is at right angles and the face veneer is a hardwood species.”
Traditionally hardwood plywood was made by “peeling” tree stems on a large rotary lathe to yield wet veneers of uniform thickness. The wet veneers were next clipped to uniform width and stacked in separate bins by grade. Then they were dried in roller dryers to remove excessive moisture as required for gluing and pressing, sorted again into different grades, repaired for natural defects and conditioned for uniform moisture content. Conditioned veneers were next coated with appropriate glue between adjacent plies, laid up in a veneer assemblies having face and back veneer as outer faces with respective grain angles along a “major axis”, centres and intermediate plies with respective grain angles perpendicular to the face and back veneers, and a core which is an inner ply with a respective grain angle in the same direction as that of the outer plies.
The face and back veneers usually consist of thin decorative veneers from, such wood species as oak, cherry, birch, maple, etc. The inner plies were usually made from aspen, spruce and other lower cost species. The glued veneer assemblies were pre-pressed as a single stack for moisture equalization and tack build-up, and pressed in a multi-opening hot press into plywood panels. The final thickness of hardwood panels varied according to the number of plies. Each pressed plywood panel was held together by the cured resin bond. After conditioning in hot stacks, these plywood panels were trimmed to size (usually 4ft ×8 ft or 1.2 m ×2.4 m in nominal size), patched for external defects, and sanded to meet the product specification requirements.
More recently a process has been developed to substitute the veneer inner plies for a homogenous wafercore (a term for waferboard used as core) to simplify and improve the consistency of the core material. By adding intermediate plies of veneer on each side, the resulting wafercore substrate provides a uniform and stable surface for thin decorative veneers.
“Waferboard” is defined in CSA 0437.0-93 as, “A panel containing layers of only randomly placed wafers.” “Wafers” in turn are defined as, “a specific type of wood flake produced as a primary function of specialized equipment (i.e. a waferizer) and having a controlled length of at least 30 mm (1¼ inch) along the grain direction, a controlled thickness, and a variable or controlled width. Each wafer is essentially flat and has the grain of the wood running predominantly in the plane of the wafer. In overall character, wafers resemble small pieces of thin veneer. Wafers purposely produced with a narrow width to facilitate alignment are called strands.”
The more recent process yields a balanced panel of stressed-skin construction for industrial application with superior strength, good dimensional stability, good machinability and decorative advantage with less thickness variation and lower formaldehyde emissions than more traditional plywood panels. Either process has similar drawbacks in requiring the drying of green veneer which requires labour and time to monitor and control the drying conditions, plus costly natural gas as a purchased fuel to provide heat for the dryers.
Either process involves a significant amount of handling in the drying of the green veneer. The purchase and operation of a dryer represents capital and maintenance costs.
It is an object of the present invention to provide a faster and more economical process for the manufacture of composite plywood.